Modern-day electronics require multiple patterned layers of electrically or optically active materials, sometimes over a relatively large substrate. Electronics such radio frequency identification (RFID) tags, photovoltaics, and optical and chemical sensors all require some level of patterning in their electronic circuitry. Flat panel displays, such as liquid crystal displays or electroluminescent displays rely upon accurately patterned sequential layers to form thin film components of the backplane. These components include capacitors, transistors, and power buses. The industry is continually looking for new methods of materials deposition and layer patterning for both performance gains and cost reductions.
Thin film transistors (TFTs) may be viewed as representative of the electronic and manufacturing issues for many thin film components. TFTs are widely used as switching elements in electronics, for example, in active-matrix liquid-crystal displays, smart cards, and a variety of other electronic devices and components thereof. The thin film transistor (TFT) is an example of a field effect transistor (FET). The best-known example of an FET is the MOSFET (Metal-Oxide-Semiconductor-FET), today's conventional switching element for high-speed applications. For applications in which a transistor needs to be applied to a substrate, a thin film transistor is typically used. A critical step in fabricating the thin film transistor involves the deposition of a semiconductor onto the substrate. Presently, most thin film devices are made using vacuum deposited amorphous silicon as the semiconductor, which is patterned using traditional photolithographic methods. Amorphous silicon as a semiconductor for use in TFTs still has its drawbacks. Thus, there has been active work to find a suitable replacement.
There is a growing interest in depositing thin film semiconductors on plastic or flexible substrates, particularly because these supports would be more mechanically robust, lighter weight, and allow more economic manufacturing, for example, by allowing roll-to-roll processing. A useful example of a flexible substrate is polyethylene terephthalate. Such plastics, however, limit device processing to below 200° C.
In spite of the potential advantages of flexible substrates, there are many problems associated with plastic supports when using traditional photolithography during conventional manufacturing, making it difficult to perform alignments of transistor components across typical substrate widths up to one meter or more. Traditional photolithographic processes and equipment may be seriously impacted by the substrate's maximum process temperature, solvent resistance, dimensional stability, water, and solvent swelling, all key parameters in which plastic supports are typically inferior to glass.
There is interest in utilizing lower cost processes for deposition that do not involve the expense associated with vacuum processing and patterning with photolithography. In typical vacuum processing, a large metal chamber and sophisticated vacuum pumping systems are required in order to provide the necessary environment. In typical photolithographic systems, much of the material deposited in the vacuum chamber is removed. The deposition and photolithography items have high capital costs and preclude the easy use of continuous web based systems.
In the past decade, various materials have received attention as a potential alternative to amorphous silicon for use in semiconductor channels of thin film transistors. The discovery of practical inorganic semiconductors as a replacement for current silicon-based technologies has also been the subject of considerable research efforts. For example, metal oxide semiconductors are known that constitute zinc oxide, indium oxide, gallium indium zinc oxide, tin oxide, or cadmium oxide deposited with or without additional doping elements including metals such as aluminum. Such semiconductor materials, which are transparent, can have an additional advantage for certain applications, as discussed below. Additionally, metal oxide dielectrics such as alumina (Al2O3) and TiO2 are useful in practical electronics applications as well as optical applications such as interference filters.
In addition, metal oxide materials can serve as barrier or encapsulation elements in various electronic devices. These materials also require patterning so that a connection can be made to the encapsulated devices.
Although successful thin films in electronic devices have been made with sputtering techniques, it is clear that very precise control over the reactive gas composition (such as oxygen content) is required to produce good quality devices. Chemical vapor deposition (CVD) techniques, in which one or more reactive gasses decompose or react to form the desired film material at a surface, can be useful routes to achieving high quality film growth. Atomic layer deposition (“ALD”) is yet an alternative film deposition technology that can provide improved thickness resolution and conformal capabilities, compared to its CVD predecessor. The ALD process segments the conventional thin-film deposition process of conventional CVD into single atomic-layer deposition steps.
ALD can be used as a fabrication step for forming a number of types of thin-film electronic devices, including semiconductor devices and supporting electronic components such as resistors and capacitors, insulators, bus lines, and other conductive structures. ALD is particularly suited for forming thin layers of metal oxides in the components of electronic devices. General classes of functional materials that can be deposited with ALD include conductors, dielectrics or insulators, and semiconductors. Examples of useful semiconducting materials are compound semiconductors such as gallium arsenide, gallium nitride, cadmium sulfide, zinc oxide, and zinc sulfide.
Advantageously, ALD steps are self-terminating and can deposit precisely one atomic layer when conducted up to or beyond self-termination exposure times. An atomic layer typically ranges from about 0.1 to about 0.5 molecular monolayers with typical dimensions on the order of no more than a few Angstroms. In ALD, deposition of an atomic layer is the outcome of a chemical reaction between a reactive molecular precursor and the substrate. In each separate ALD reaction-deposition step, the net reaction deposits the desired atomic layer and substantially eliminates “extra” atoms originally included in the molecular precursor. In its most pure form, ALD involves the adsorption and reaction of each of the precursors in the complete absence of the other precursor or precursors of the reaction. In practice in any process it is difficult to avoid some direct reaction of the different precursors leading to a small amount of chemical vapor deposition reaction. The goal of any process claiming to perform ALD is to obtain device performance and attributes commensurate with an ALD process while recognizing that a small amount of gas phase nucleation can be tolerated.
In ALD processes, typically two molecular precursors are introduced into the ALD reactor in separate stages. The details of such stages and molecular precursors useful in each are explained in [0016]-[0034] of U.S. Patent Application Publication 2009/0081827 (Yang et al.) that is incorporated herein by reference along with the references mentioned in these paragraphs.
U.S. Patent Application Publication 2005/0084610 (Selitser) discloses an atmospheric pressure atomic layer chemical vapor deposition process that involve separate chambers for each stage of the process and a series of separated injectors are spaced around a rotating circular substrate holder track.
A spatially dependent ALD process can be accomplished with other apparatus or methods described in more detail in WO 2008/082472 (Cok), U.S. Patent Application Publications 2008/0166884 (Nelson et al.), 2008/0166880 (Levy), 2009/0130858 (Levy), 2009/0078204 (Kerr et al.), 2009/0051749 (Baker), 2009/0081366 (Kerr et al.), and U.S. Pat. Nos. 7,413,982 (Levy), 7,456,429 (Levy), and 7,572,686 (Levy et al.), all of which are hereby incorporated by reference in their entirety. These publications described various attempts to overcome one of the difficult aspects of a spatial ALD system, which is undesired intermixing of the continuously flowing mutually reactive gases.
There is growing interest in combining ALD with a technology known as selective area deposition (or “SAD”) in which a material is deposited only in those areas that are desired or selected. Sinha et al. [J. Vac. Sci. Technol. B 24 6 2523-2532 (2006)] have remarked that selective area ALD requires that designated areas of a surface be masked or “protected” to prevent ALD reactions in those selected areas, thus ensuring that the ALD film nucleates and grows only on the desired unmasked regions. It is also possible to have SAD processes where the selected areas of the surface area are “activated” or surface modified in such a way that the film is deposited only on the activated areas. There are many potential advantages to selective area deposition techniques, such as eliminating an etch process for film patterning, reduction in the number of cleaning steps required, and patterning of materials which are difficult to etch. One approach to combining patterning and depositing the semiconductor is shown in U.S. Pat. No. 7,160,819 (Conley et al) that describes materials for use in patterning zinc oxide on silicon wafers.
A number of materials have been used by researchers as deposition inhibitor materials for selective area deposition. Sinha et al., referenced above, used poly(methyl methacrylate) (PMMA) in their masking layer. Conley et al. employed acetone and deionized water, along with other process contaminants as deposition inhibitor materials.
U.S. Patent Application Publications 2009/0081827 and 2009/0051740 (both noted above) describe the use of crosslinkable organic compounds or polymers, such as organosiloxane polymers, as deposition inhibitor materials, in ALD processes to provide various electronic devices. These crosslinkable materials are generally coated out of organic solvents. There is a need to avoid the use of organic solvents in providing deposition inhibitors when fabricating various useful devices using chemical vapor deposition techniques such as ALD.
The problem with these previously used materials is that they rely upon polymers that are soluble only in aggressive organic solvents. Aside from health and environmental concerns, the use of aggressive organic solvents is difficult in a large scale manufacturing process. Among these disadvantages include: (a) cleanup of coated materials must be done with similar organic solvents, leading to increased solvent usage, and (b) many of the printing technologies proposed for printed electronics leverage elastomer printing plates that swell upon contact with aggressive organic solvents. Therefore, there has been a need for selective deposition inhibitors that are soluble in environmentally friendly solvents, principally water and alcohols, and can be applied as aqueous formulations.